Frequency response control means for high-frequency amplifiers



O. F. CHENEY FREQUENCY RESPONSE CONTROL MEANS 'July 21, 1953 FORHIGH-FREQUENCY AMPLIFIERS 2 Sheets-Sheet l Filed Sept. 6, 1950 INVENToROL/ l/R F. Cliff/76) asf/:7:5

July 21, 1953 Q F, CHENEY 2,646,471

FREQUENCY RESPONSE CONTROL MEANS FOR HIGH-FREQUENCY AMPLIFIERS FiledShept. 6, 1950 2 Shees--Shee'l 2 INVENToR'. @ZH/? E (Hf/76V PatentedJuly 21, 1953 FREQUENCY RESPONSE CONTROL MEANS FOR HIGH-FREQUENCYAMPLIFIERS Oliver F. Cheney, Philadelphia, Pa., assigner to PhilcoCorporation, Philadelphia, Pa., a corporation of PennsylvaniaApplication September 6, 1950, Serial No. 183,359

The invention herein' described and claimed ref lates to high-frequencyamplifiers, and particu- 3 Claims. (Cl. 179-?171) larly to means formodifying 'the frequency response of a multi-stage stagger-tunedamplifier l system in response to variations in signal strength.

The problem -to'which the present 1nvent1on is directed is of particularimportancel in television receivers.

As is shown graphically in Figure 2, to which detailed reference willhereinafter be made, the picture I.F. carrier, in most televisionreceivers at least, is located at approximately the 50%amplitude-response point at the upper-frequency end of the overallcharacteristic of the multistage stagger-tuned I.F. amplifier system.The sound I.-F. carrier is located on a relatively level shelf atapproximately the 5% amplitude-response point at the lower-frequency endof the curve, as also shownin Figure 2. When such a receiver is operatedin an area where the signal strength is weak, the reproduced image maybe largely obscured by noise signals, since a ysubstantial portion ofthe noise signals Willbe located on the 100% amplitude-response portionof the characteristic. In such case, the main areas ofthe reproducedimage could be strengthened by shifting the picture I.F. carrier towarda lower frequency, Vsay from 26.6 megacycles to 25.6 megacycles, for todo so would move the carrier up the slope of the characteristic from the50% to about the 90% or 100% amplitude-respense point. This wouldimprove the signal-tonoise ratio with respect to the low-frequencymodulation components of the picture signal. However, if the operatordoes shift the picture I.-F. carrier in the manner suggested, he does soby mistuning the local oscillator and, as a result, boththehigh-frequency modulation components of the picture signal and thesound signal will be impaired rather than improved. Where the signal isweak, the sacrice of high denition or fine picture detail for increasedstrength in the main areas of the picture is advantageous but thesacrifice of all or a substantial amount of the sound outputisobjectionable. In many prior art television receivers, operated in areasof weak signal, when the operator adjusts his fine-tuning control tostrengthen the picture, he loses the sound completely. For, when heshifts the picture I.-F. carrier up the slope to the 90-100% level,heunavoidably moves the sound I.F. carrier off the shelf and down theslope at the lower-frequency end of the characteristic.

Accordingly, it is a principal object of the present invention toimprove the sensitivity of a television receiver by providing meansoperative automatically .011 Wink Signals toimprove the signal-to-noiseratio with respect tothe low-frequency modulation components of thepicture signal without, however,l adversely aiTecting the sound output.

Another object is to'prov'i'de; in' the 1;-F. ampliier system ofa'television receiver whose picture I.F. carrier frequency is higherthanthe sound I.-F. carrier frequency, means responsive to variations inapplied .automatic-gain-ccntrol bias for boosting automatically thelow-frequency modulation lcomponents of the picture signal when thesignal strength is weak, without impairing the sound output.

The foregoing objects are achieved, inV accordance with the presentinvention, by providing, in a selected stage of the multi-stagestagger-tuned I.-F. amplier system, means automatically responsive toVariations in applied automatic-gaincontrol bias to boost the responseat the upperfrequency end of the I.-F. amplifier systems characteristic.The proper tuning point for the picture .-F. carrier remains unchangedinfre-v quency but theamplitude-response at the picture the response isnormal, being such that the picture I.-F. carrier is located at aboutthe 50% amplitude-response point on the characteristic. Thellower-frequency end of the response characteristic is not appreciablydisturbed. byV the action of the means provided by the present inventionandthe sound output is not-impaired.

The means provided by the present invention. lcomprise a capacitorandcathode load impedance, of selected magnitudes connected in that stageof the stagger-tuned I.-F. amplier system which supports theupperfrequency end ofthe response curve. The capacitor is connectedbetween the grid and the `cathode end or" the cathode load im-V pedance.The cathode load `impedance is preferably a resistor. c j y ln theabsence of the capacitor and` cathode load impedance provided lbythepresent invention, when the signal strength becomes weaker,A

the applied autcmatic-gaincontrol bias decreases, the gain and mutualconductance of the particular tube increases, the input capacitanceofthe tube increases, and the response vat the upper-frequency end ofthe characterisic decreases due to the resultant change in tuning of theinput circuit. When, however, the aforesaid ca- .to the resultant changein tuning of the input circuit.

The foregoing will be better understood from a consideration of thefollowing detailed description taken together with the accompanyingdrawing wherein:

Figure l is a representation, partly diagrammatic, partly schematic,showing a television receiver which incorporates the improvement of thepresent invention, and y Figure 2 shows idealized frequency-responsecurves for a stagger-tuned I.F. amplifier system, indicating the mannerin which the response characteristic is altered, on weak signals, by theaction of the means provided by the present invention.

While the present invention is also applicable to the type of televisionreceiver now `considered conventional, wherein separate I.F. channelsare provided for picture and forsound, it will be convenient to describethe invention as applied to an intercarrier-sound type of televisionreceiver, and this type of receiver has been shown in Figure 1.

Referring now to Figure l, there is shown a portion of a televisionreceiver of the intercarrior-sound type comprising antenna l0, R.F.amplier I'I, frequency converter I2, video and sound I.F. amplifierstages I4 to I6, video detector and automatic-gain-control circuit i8,video amplifier I9, video output and D.C. restorer 20, and picture tube2|. The sound signals may be taken oi at video amplifier I9 and passedthrough the usual sound channel, which may comprise the usual sound I.F.amplifier, frequency-modulation detector, audio amplier, output stage,and loud speaker, none of which is shown in the drawing. The receiver ofFigure l also includes, as is conventional, a synchronizing-signalseparator circuit, and vertical and horizontal deflection circuits.

In intercarrier-sound television receivers, as well as in theconventional type of receiver employing separate I.F. channels forpicture and for sound, it is customary to locate the picture I.F.carrier and the sound I.F. carrier on opposite slopes of thefrequency-response characteristic of the I.F. amplifier system. As showngraphically in Figure 2, the picture I.F. carrier is customarily locatedat about the 50% amplitude-response point on the slope at theupperfrequency end of the characteristic. In Figure 2, the picture I.F.carrier is indicated to be 26.6 megacycles. The sound I.F. carrier,which is a fixed number of megacycles below the picture I.F. carrier, islocated at the lower-frequency end Vof the characteristic, customarilyon a relatively fiat shelf at about the amplitude-response level. InFigure 2, the sound I.F. carrier is indicated to be 22.1 megacycles, 4.5megacycles below the picture I.F. carrier.

It will be seen that if, for the purpose of improving the picture, theoperator adjusts his local oscillator so as to move the picture I.F.carrier toward a-lower frequency, i. e. toward the left in Figure 2, sayfrom 26.6 megacycles to 25.6 megacycles, the main arcas 0f. the imagewould be substantially improved since the picture I.F. carrier wouldhave been moved from its normal 50% amplitude-response location to theSil-% amplitude-response region and the low-frequency modulationcomponents of the picture signal would have become stronger relative tothe noise signals. However, the highfrequency modulation componentswould have been made weaker, for they would have been moved down theslope at the lower-frequency end of the characteristic. And, what, ismore important, since the frequency-difference between the picture andsound I.F. carriers is xed, the sound I.F. carrier will necessarily havebeen moved to a lower frequency, i. e. from 22.1 megacycles to 21.1megacycles, in the present example. no longer on the shelf; it has beenmoved off the shelf and down the slope and the sound output has beenreduced to below audibility, or at least to an objectionably low level.

To` vavoid the undesirable results indicated above, the concept of thepresent invention is to provide means whereby, in lieu of shifting thepicture I.F. carrier, and hence also the sound I.F. carrier, toward alower frequency, to the left in Figure 2, the upper-frequency end of thefrequency-response characteristic is, in response to variations inapplied automatic-gain-control bias, automatically boosted at the higherfrequencies in the manner indicated by the dotted line 13 in Figure V2.The sound I.F. carrier remains undisturbed on its shelf.

The foregoing is accomplished, in accordance with my invention, bychanging automatically the resonant frequency of the particular I.F.amplifier' stage which supports the lupper-frequency end of the I.F.amplifier systems overall frequency-response characteristic. In Figure2, the-second I.F. amplifier is indicated as supporting theupper-frequency end of the characteristic. On strong signals, the secondI.F. amplifier is tuned say to 25.6 megacycles, but on weaker signals,in response to reduced bias, the means provided by the present inventionshifts the resonant frequency of the second I.F. amplifier toward say26.3 megacycles, as indicated in Figure 2 by the dotted line fil.

It is to be particularly noted that the shifting of the resonantfrequency of the second I.F.

.amplifier stage toward a higher frequency, in

response to a reduction in applied automaticgain-control bias, is thereverse of that which tends inherently to occur. On weak signals, thereceiver operates at substantially full gain and the input capacitanceof each amplifier stage is greater than when the receiver is operatingat reduced gain. Consequently, when, in the presence of weak signals,the gain of the amplifier system is increased by the operation of theautomatic-gain-control system, the frequency response of the amplifiersystem tends to decrease at the upper-frequency enol of thecharacteristic. The present invention, however, provides means forincreasing, rather than decreasing, the response at the upper-frequencyend when the gain of the amplier system is increased.

The means employed by the present invention preferably comprise anunbypassed cathode load resistor and a fixed capacitor. 'Ihe capacitoris connected between the grid of the tube and the cathode end of thecathode load resistor. Both are connected in that particular stage ofthe I.F. amplifier system which supports the upperfrequency end of theoverall characteristic. 11n

Thus, the sound I.F. carrier is- Figure l, the elements are shown to beconnected mosca) f where Rzthe resistance (in ohms) of the cathode loadlresistor.

Cizthe capacitance (in farads) of the external grid-cathode capacitor.

Cgkzthe grid-cathode interelectrode capacitance (in farads) when thetube is cut olf. A

gmzthe'transconductance (in .mhos) of the tube for a particularoperating point.

ACzthe change (in farads) in grid-cathode in.-

terelectrode capacitance when the transconductance is gm as comparedwith the capacitance when the tube is cut off.

It will be seen that the above relation may be satised either by keeping(C'i-f-Cgk) small and making R. adequately large, or by keeping R smalland making (C14-Cyr) adequately large, or by a suitable combination ofintermediate values. Since the employment of a cathode load resistorcauses an unavoidable loss in gain, proportional to the size of theresistor, it is preferable to keep R small and to add a physicalcapacitor C1 of sufciently large value to produce the desired results.In practice, I prefer that the cathode load resistor be given that valuewhich will substantially compensate for variations inthe gridcathodeinterelectrode capacitance of the tube. This value may bedetermined bythe relation AC Ckgm where R., Cgk, AC and gm are as defined previouslyabove. With R having the value determined by the above relation, thephysical gridcathode capacitor C1 becomes the element which effects theboostin the frequency response at `the appears to be that, when inresponse to Weakv signals, the mutual conductance of the tube increases,the potential differencebetween the grid and cathode decreases, lesscurrent flows through the grid-cathode capacitor, and the effectiveinput capacitance of the stage decreases. Thus, the frequency at whichthe input network of the stage is resonant is higher 'on' weak signalsthan on strong signals. This. is shown graphically in Figure 2 where thedotted line il indicates the response of the second I.F. amplifier stageon weak signals and the solidline curve 2 indicates the response onstrong signals. The effect on the overall characteristic of shifting theresonant frequency of the particular I.F. amplifier stage 'whichsupports'the upper-frequency end of the characteristic is also showngraphically in Figure 2. There, the dotted line 40 indicates the mannerin which theoverall response is boosted on weak signals.

In Figure l, I have shown, for purposes of illustration, typical valuesof grid-cathode capacitor and of cathode resistor which may be Vemployedto achieve the desired boost inresponse at the picture I.F. carrierfrequency. Tube ,3G is a BCBG having a cold grid-to-cathodeinterelectrode capacitance of 4.8 ,L /.cf. and ahot capacitance of 6.3Miha variation of 1.5 auf'. The transf conductance of the tube at fullgain lis ofthe order of 6000 ambos. The cathode resistor y32 is 39 ohms.-Capacitor 33 has a. value of 12 auf. Setting the above values to thesymbols used in the formulae hereinbefore given, we have ym=6000 arnhos.

Thus, we see that thevvalues of KVR, and` C1 "given, by way of example,in the circuit of Figure l,

satisfy the first formula.

Applying the appropriate valuesl to the second formula, which statesthaty R-should havea value substantially equal to Y AC' ,Y Cskgm.

we get Solving, we ndthat R should be substantially` equal to In otherwords, we find that-thc cathode :load

resistor R should have a value of the order of 52 ohms. l In the circuitof Figure l, which represents a commercial embodiment of the invention,a 397-*y ohm resistor is used. It will be understood that this valueA issubstantially equal to that cated by thefsecond formula.

Having described my invention, I claim:V l. In a high-frequencyamplifier stage; a tube having at least cathode, control vgrid and anodeindielectrodes and a tuned network which includesas a component theinherent input capacitance of said tube, .said input capacitance beingsubject to variation as a function of control-grid ybias whereby thefrequency response of said AC MCH-Cab;-

Where R=the resistance (in ohms) of the cathode load resistor,

C1=the capacitance (in farads) of the external grid-cathode capacitor,Cgk=the grid-cathode interelectrode capacitance (in farads) when thetube is cut oli, ym=the transconductance (in mhos) ci the tube for aparticular operating point, AC=the change (in farads) in grid-cathodeinterelectrode capacitance when the transconductance is gm as comparedwith the capacitance when the tube is out oir,

and R having a ,value substantially equal to that which will satisfy therelation- 2. In a television receiver; an intermediatefrequencyamplifier system comprising a plurality of stagger-tuned stages each ofwhich includes a tube having at least cathode, control grid and Vanodeand a tuned input network which includes as a component the inherentinput capacitance of said tube, said amplier system having a pass bandwhich includes, near the upperfrequency end, the picture I.F. carrierfrequency, one of said stages being tuned to a frequency near saidupper-frequency end; an automaticgain-control system for developing acontrol bias and applying said bias `to at least some of said stages ofsaid amplier system including said stage tuned to a frequency near saidupperfrequency end, thereby to vary the gain of the stages to'which saidbias is applied in response to variations in signal strength; and meansresponsive to variations in control bias for modifying automatically thepass band of said amplier system in such manner that the response toweak signalsY at the picture I.F. carrier frequency is substantiallyincreased, said last-named responsive means comprising a capacitorconnected between the grid and cathode of the tube of said'amplier stagewhich is tuned to a frequency near said upper-frequency end and acathode load resistor connected between said lastnamed cathode andground, said grid-to-cathode capacitor having a value which will satisfythe relation- VA R(C1+C.k g-C where R=the resistance (in ohms) of thecathode load resistor,

C1=the capacitance (in farads) of the external grid-cathode capacitor,

Cgk=the grid-cathode interelectrode capacitance (in farads) when thetube is cut off,

ym=the transconductance (in mhos) of the tube for a particular operatingpoint,

AC=the changeV (in farads) in grid-cathode interelectrode capacitancewhen the transconductance is gm as compared with the capacitance whenthe tube is out off,

and R having a value substantially equal to that which will satisfy therelation- 3. In an amplifier system; a plurality of stagger-tunedamplier stages each of which includes a tube having at least cathode,control grid and anode and a tuned input network which includes as acomponent the inherent input capacitance of said tube, one of saidstages being tunedto a frequency near the upper-frequency end of thepass band of said system; an automatic-gain-control system fordeveloping a control bias and applying said bias to at least some ofsaid stages of said amplifier system including said stage tuned to afrequency near said upperfrequency end, thereby to vary the gain of thestages to which said bias is applied in response to variations in signalstrength; and means responsive to variations in control bias formodifying automatically the pass band of said amplier system in suchmanner that the response to Weak signals at frequencies near saidupperfrequency end of said pass band is substantially increased, saidlast-named responsive means comprising a capacitor connected between thegrid and cathode of the tube of said ainplier stage which is tuned to afrequency near said upper-frequency end and a cathode load resistorconnected between said last-named cathode and ground, saidgrid-to-cathode capacitor having a value which will satisfy therelationwhere R=the resistance (in ohms) of the cathode load resistor,

C1=the capacitance (in farads) of the externa grid-cathode capacitor,Cgk=the grid-cathode interelectrode capacitance (in farads) when thetube is cut o, gm=the transconductance (in nihos) of the tube for aparticular operating point,

AC=the change (in farads) in grid-cathode interelectrode capacitancewhen the transconductance is gm as compared .with the capacitance whenthe tube is cut ofi,

and R having a value substantially equal to that which will satisfy therelation-

